EP0228111B2 - Process for the removal of noxious matter from waste gases - Google Patents

Abstract

1. A process for the dry removal of polluants from the exhaust gases of combustion processes in the processing of stones and earths, characterised in that the exhaust gases are brought into contact with a sorbent in the form of solids particles at 50 to 100 degrees C in a solids circulation system formed by a fluidised bed reactor (1), a solids separator (2) and a recycling line (3), and that the laden sorbent is completely or almost completely supplied to a cement production process.

Description

The invention relates to a method for the dry removal of pollutants from the exhaust gases of cement production processes. In order to comply with the applicable environmental protection regulations, plants for the cement manufacturing industry are increasingly being equipped with facilities for the separation of harmful gases.

In the known wet cleaning methods, the exhaust gases are usually treated after dedusting with a circulating washing liquid and thereby cooled to such an extent that reheating to about 20 ° C. above the dew point is necessary in order to avoid corrosion damage. The resulting amount of reaction products is usually too large to be returned to the manufacturing process as a whole. Wet cleaning processes are therefore usually associated with high investment and operating costs, which often prevent rapid implementation of exhaust gas cleaning.

When manufacturing cement, attempts have already been made to dry-clean the exhaust gases at 300 to 850 ° C. It has been found, however, that the exhaust gases at temperatures above 500 ° C often contain less than 100 mg / Nm ³ of the pollutant S0 ₂ in the foreground, because the majority of the sulfur introduced into the cement manufacturing process with the fuel in this Temperature range is sorbed by raw meal and integrated into the clinker.

In the cement manufacturing process, however, sulfur is also introduced into the firing process by the raw meal. This sulfur originates partly from organic or sulfidic sulfur compounds, which decompose even at low temperatures and lead to the release of S0 ₂ during the preheating of the raw meal, whereby S0 ₂ contents up to 6000 mg / Nm ³ can occur. Since there is practically no sorption of S0 ₂ by the raw material at the exhaust gas temperatures usual in the cement process, an exhaust gas desulfurization device must also be provided for cement production plants, although the sulfur introduced with the fuel is only contained to a small extent in the exhaust gas due to the process.

Dry cleaning at temperatures of 200 to 400 ° C is not particularly effective regardless of the sorbent used and regardless of the ratio of sorbent to the amount of pollutant. In general, only less than 60% of the pollutants can be removed from the exhaust gas if one does not want to switch to expensive and, due to the usually high dust accumulation, only reliable operational cleaning methods with catalysts.

It is therefore the task of proposing a process for the removal of pollutants from the exhaust gases of combustion processes in the cement manufacturing industry which is generally applicable and more economical than the known processes.

This object is achieved according to the invention in that the exhaust gases are brought into contact with a sorbent in the form of solid particles at 50 to 100 ° C. in a solid circulation system formed from a fluidized bed reactor (1), solid separator (2) and return line (3), whereby a mixture of raw meal and hydrated lime is used as the sorbent, the proportion of the hydrated lime covers about 50% of the stoichiometrically required calcium requirement, the temperature in the fluidized bed reactor (1) is regulated by spraying in water and the loaded sorbent is returned to the cement production process.

With the method according to the invention, not only the contact time, which is often too short for other dry cleaning methods in the lower temperature range, can be increased, but a product is also created that does not require any special further processing or landfilling, as a result of which the economic viability of conventional methods is often very burdened.

Before entering the solid circulation system, the exhaust gases expediently have a temperature of 50 to 100.degree. Since an end dust separator is usually present in plants for firing stones and earth, this can be used as a solid separator of the solid circulation system.

The main advantage of the method according to the invention is that materials such as raw meal and hydrated lime are used as sorbents for the production of cement and are subsequently fed back into a cement production process.

Finally, it is provided that the temperature in the fluidized bed reactor is regulated to an optimum value by spraying in water and the addition of sorbent as a function of the pollutant content in the clean gas.

Further details are explained using the example of the desulfurization of exhaust gases from a cement production process and the process flow diagram shown in simplified form in FIG. 1.

The solid circulation system consisting of the fluidized bed reactor 1, the solids separator 2 and the return line 3 is fed via line 4 with spray water, via line 5 with sorbents and via line 6 with the exhaust gas to be desulfurized. The largely desulfurized exhaust gas leaves the fluidized bed reactor 1 at the top, is freed of dust in the solids separator 2 and reaches the chimney via the blower 8 and the line 9. Under the solids separator 2, a conveyor is arranged with which the separated dust transports to the return line 3 tiert and finally abandoned in the fluidized bed reactor 1. A subset of the circulating solid particles is discharged from the solid circulation system via line 7 and fed to the cement production process at a suitable point.

Individual components contained in the solid, e.g. Heavy metals can be discharged separately from the other substances via line 10 after selective separation.

Depending on whether and which heat recovery devices (waste heat boiler, hot water preparation, mill) are provided, the exhaust gases occur at a temperature of 100 to 450 ° C. Their dust content is up to 100 g / Nm ³ if they are not passed through a mill. In the latter case, the dust loading can be up to 1000 g / Nm ³ and it may be useful to dedust dust using an electrostatic filter or mechanical separator in order to be able to limit the solids circulation in the solids circulation system to the optimum amount for desulfurization. If necessary. In such cases, the separate addition of sorbents can be dispensed with in whole or in part. The dust content of the exhaust gases is reduced to less than 50 mg / Nm ³ in the solids separator.

The S0 ₂ content in the exhaust gas is up to 6000 mg / Nm ³ depending on the raw material composition. With the method according to the invention, it can be reduced to values of less than 50 mg / Nm ³ in the clean gas. The amounts of S0 ₂ permitted by law are currently 400 mg / Nm ³ in Germany and 500 mg / Nm ³ in Switzerland.

Another advantage of the process is that to improve economy in the combustion processes, fuels can also be used which contain heavy metals, including class 1 pollutants, such as cadmium, thallium, mercury and the like. These pollutants are mostly bound to the particularly fine-grained solid particles in the fluidized bed reactor and can be separated in a classifying solid separator, e.g. a multi-field electrostatic precipitator, separated almost completely from the other pollutants and discharged independently of the reaction products returned to a cement manufacturing process.

The solid particles discharged in this way have a high concentration of pollutants on the one hand, but on the other hand represent only a small amount of the total reaction products. They are returned to the cement production process at a suitable point, since they are themselves stable and water-insoluble. In cement production, it makes sense to mix them in during clinker or cement grinding. With the method according to the invention, 90% of the pollutants mentioned can be separated from the exhaust gases and disposed of in an environmentally friendly manner.

The method according to the invention was used on a trial basis in a cement production plant. The sulfide content of the raw material was 0.3 to 0.4%, based on the weight of the raw material, S0 ₂ contents in the examined partial exhaust gas stream being measured up to 3600 mg / Nm ³ when the method according to the invention was not used has been. When using the method, the S0 ₂ content could be reduced to values well below 400 mg / Nm ³ . Raw meal was used as the sorbent, to which lime hydrate was mixed in such an amount that it covered about 50% of the stoichiometrically required calcium requirement.

Claims (5)

1. A process for the dry removal of pollutants from the exhaust gases in cement production processes, characterised in that the exhaust gases are brought into contact with a sorbent in the form of solids particles at 50 to 100°C in a solids circulation system formed by a fluidised bed reactor (1), a solids separator (2) and a recycling line (3), wherein a mixture of raw meal and slaked lime is used as a sorbent, the proportion of slaked lime covers approximately 50% of the stoichiometrically required calcium requirement, the temperature in the fluidised bed reactor (1) is controlled by the spraying-in of water and the laden sorbent is recycled into the cement production process.

2. A process according to Claim 1, characterised in that the exhaust gases are cooled to 50 to 100°C before entering the solids circulation system.

3. A process according to one of Claims 1 or 2, characterised in that the final dust collector which is usually provided in a burning plant is used as a solids separator (2).

4. A process according to Claims 1 to 3, characterised in that the addition of sorbent is controlled dependent on the pollutant content of the pure gas.

5. A process according to Claims 1 to 4, characterised in that fuels are used which contain heavy metals wherein the product formed by the binding of these pollutants to fine-grained solids particles of the sorbent in the fluidised bed reactor (1) is removed almost completely separately and discharged independently from the other pollutants in a classifying solids separator (2) and is recycled into the cement production process and is added during the grinding of clinker or cement.

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